1. Field of Invention
The present invention relates to a proximity effect correction method for fabricating a mask. More particularly, the present invention relates to a quantum proximity effect correction (QPEC) method that integrates electron beam proximity effect correction method and the optical proximity effect correction method in the fabrication of a mask.
2. Description of Related Art
Following the recent progress in integrated circuit fabricating techniques, line resolution finer than 0.25 .mu.m becomes quite common. As demands for speed and quality of semiconductor devices continue to escalate, technologists in the semiconductor industry are striving towards improved methods for solving problems caused by a low resolution. Photolithographic processing is a major step in the fabrication of semiconductor. In photolithographic processing, a defined pattern is transferred to a silicon wafer. The process of transferring the pattern to the silicon wafer comprises the steps of providing a mask having a defined pattern in it. Then, through exposure to light and subsequent development, the pattern in the mask is transferred to a photoresist layer over a wafer. Thereafter, the photoresist layer is used as a mask for etching the layer or layers below.
Since the mask is a critical component for carrying out the transfer of circuit patterns to the wafer, it must be carefully prepared each time before patterning the photoresist layer through light exposure. If a mistake is made in the fabrication of the mask, most probably the whole batch of wafers may have to be scrapped. Therefore, in order to speed up the fabrication process and minimize errors, most equipment and processes in integrated circuit manufacturing is under computer control.
Because an electron beam photolithographic technique can have a maximum resolution of about 0.375.+-.0.05 .mu.m while an optical laser photolithographic technique can have a resolution at most around 1.+-.0.3 .mu.m, most integrated circuit photo mask production that fabricates masks with feature line smaller than about 0.25 .mu.m all use the electron beam photolithographic technique to carry out exposure. Although the resolution in using an electron beam photolithographic technique is considerably higher, scattering often occurs when a pattern is defined. Hence, ultimate resolution and line width is difficult to control. The scattering of electron beam limits the ultimate resolution of the line width, and this phenomenon is known as proximity effect. This proximity effect can be corrected by using an electron beam proximity effect correction method. The electron beam proximity effect correction method can be used to correct proximity effect problem in the mask when the mask tooling operation is carried out. However, although the mask is corrected for its proximity effect by the electron beam proximity effect correction method during mask tooling operation, problems arising out of optical proximity effect still need to be solve due to the application of a stepper machine to project the image from the mask onto the wafer.
When light is shone by a stepper machine through a mask onto a wafer, optical proximity effect will occur due to the diffraction and interference of light. Hence, an optical proximity effect correction method must be used to correct the image. However, using softwares to correct the stepper machine often requires a large computer-aided design (CAD) image data files, and so precise pattern transfer is difficult to achieved.
At present, most integrated circuit factories apply the electron beam proximity effect correction method on mask tooling and using corrected electron beam pattern data files for performing mask tooling. Thereafter, an optical proximity effect correction method is used to correct the corrected electron beam pattern data files before performing mask tooling for exposure by the stepper. However, the use of these two methods repeatedly to correct the image not only will distort the pattern, but will also create a large data file for storing the pattern. For example, FIGS. 1a through 1e are a series of diagrams showing the defective transfer of pattern by a conventional photolithographic processing.
In FIG. 1a, a desired pattern for putting on the mask is shown. For illustration purposes, the pattern includes a long line 100 at top and a grid gate pattern 102 below. First, the CAD image file data are transmitted to the mask. As shown in step (a), if an electron beam proximity effect correction method is used to correct the image, the result is that the whole pattern defined in the CAD image data is transferred to the mask intact as shown in FIG. 1b. On the other hand, if step (b) is chosen where no electron beam proximity effect correction is applied, the final image is distorted as shown in FIG. 1c. In other words, when the pattern defined in the CAD image data is transferred to the mask, problems such as gross pattern distortion, broken lines and shrinkage often arise. As illustrated in FIG. 1c, the long line 100 as originally defined in the pattern is transformed into two parts, namely, a broken and shorter line 100a and a disappearing top line 100b, while the grid gate as originally defined in the pattern will turn into a shrunk grid gate pattern 102a.
When the defined pattern on the mask is again transferred to the wafer by shining light through a mask using a stepper, optical proximity effect problem is encountered. Step (c) shows the effect of using an optical proximity effect correction method to correct the image during light exposure of wafer. The defined pattern on the mask is transferred to the wafer surface as shown in FIG. 1d. Alternatively, if no optical proximity effect correction method is used as in step (d), the final pattern is as shown in FIG. 1e. The transferred pattern from the defined pattern on the mask will be distorted. In other words, when the pattern defined on the mask is transferred to the wafer, problems such as gross pattern distortion, broken lines and shrinkage often arise. As shown in FIG. 1e, a long line 100 as originally defined in the pattern is transformed into two parts, namely, a broken and shorter line 100c and a disappearing top line 100d, while the grid gate as originally defined in the pattern will turn into a shrunk grid gate pattern 102b. Consequently, both the electron beam proximity effect correction method and the optical proximity effect correction method must be used together in photolithographic processing operation.
However, using optical proximity effect correction method to correct the exposure by a stepper and correcting the defined pattern on the mask at the same time needs a large amount of memory to store the pattern in a CAD data file. Furthermore, the mask also needs to perform a subsequent electron beam proximity effect correction process to correct the mask tooling of electron beam. Because of repeated corrections, the image pattern will be distorted. Moreover, a very large image data file needs to be kept and the time necessary to bring about the correction is long.
In light of the foregoing, there is a need to improve the proximity effect correction method for a mask.